This invention relates generally to combined thermal and radiation shielding apparatus and more specifically relates to apparatus for providing thermal and radiation shielding for components within a well logging instrument.
In the oil and gas exploration industry it is well known to determine parameters of subsurface earth formations by surveying or logging a borehole penetrating such formations by traversing the borehole with one or more of various types of logging instruments. Some species of these logging instruments use radiation-sensitive devices to determine formation parameters, such devices requiring appropriate use of radiation shielding to assure accuracy of the determined parameters. For example, in a not uncommon embodiment of a gamma-gamma density logging instrument used to determine the bulk density of the formations, the formations are irradiated with gamma radiation emitted by a source supported adjacent to the formations in a skid pad designed to generally follow the contours of the borehole sidewall. The scattering of the gamma radiation caused by the formations is determined by measuring the intensity of scattered gamma radiation at two dissimilar longitudinal distances from the radiation source by two gamma ray detectors also supported in the skid pad. Radiation shielding is provided within the skid pad around the source to generally collimate the gamma radiation emitted by the source and insure that such radiation is directed into the formations surrounding the borehole, and also is provided around the detectors to insure that they are impinged only by radiation scattered within, and coming directly from, the formations adjacent the skidpad. This radiation shielding requires the distribution of a mass of material around the source and detectors in such proportions that the thickness of the material within the path of the radiation combined with the density of the material, serve to essentially block the passage of the radiation therethrough. The intensity of the radiation measured by the detectors may then be correlated with instrument calibration data to yield an indication of the bulk density of the formations.
In conducting well logging operations it is not uncommon to encounter borehole temperatures in excess of 260.degree. C. Therefore, it is necessary for logging instruments utilized in such high temperature environments either to consist of components capable of operating at such temperatures or to provide adequate thermal shielding to maintain an acceptable thermal environment for the components. One means to accomplish such thermal shielding is to place the components requiring thermal protection within a Dewar flask, such flask also containing a volume of a eutectic compound to serve as a heat sink. As the instrument is lowered into a borehole and the ambient temperature surrounding the instrument increases, heat will gradually leak into the Dewar flask. Once the temperature reaches the eutectic temperature of the eutectic heat sink however, the heat will be generally absorbed by the eutectic substance as it undergoes its isothermal transformation, thereby causing the temperature within the flask to remain generally stable until the majority of the heat sink material has melted. Components within the flask may therefore be maintained at a temperature substantially equivalent to the melting temperature of the eutectic compound for a given period of time dependent upon the quality of the flask and the volume of eutectic material used as a heat sink.
The size of a logging instrument is often an important design consideration therefor as it may affect the ability of the instrument to traverse the borehole in the desired manner. This is particularly true in cases such as the gamma-gamma density logging instrument discussed earlier herein, wherein components are supported within a skid pad which is preferably of as generally minimal length as is practicable so as to facilitate the ability of the skid pad to follow the borehole contours while providing sufficient spacing between source and detectors for accurate measurements. The greater the length of the skid pad, the greater the tendency for the skid pad to lose contact with the borehole sidewall as it traverses washouts or anomalies therein, thereby leading to inaccuracies in the determined formation density. Because of the spatial restrictions upon logging instruments, and because of the absolute requirement of a mass of radiation shielding for some components contained therein, some logging instruments, of necessity, have been adapted to operate with minimal or no thermal shielding for such components because the volume of thermal ballast required to provide an effective heat sink along with the necessary mass of radiation shielding would require a volume in excess of practical limits. For example, prior art methods of gamma-gamma density logging have often relied upon the use of Geiger counter tubes capable of operating at the high temperatures encountered within boreholes. Notwithstanding the operation of these tubes at borehole temperatures, the life expectancy and reliability of the Geiger counter tubes can typically be significantly increased by shielding the tubes from the hostile thermal environment of the borehole. Further, it would often be preferable to facilitate the use of scintillation counters within such logging instruments, rather then Geiger counter tubes, because of the increased sensitivity to radiation of the scintillation counters and the accompanying improved resolution of measurement achievable through use thereof. Additionally, in certain types of logging instruments it is desirable to measure only radiation of certain predetermined energy levels, such selective energy level detection being beyond the capability of conventional Geiger counter tubes. Scintillation counters, however, will typically operate satisfactorily only at temperatures significantly lower than those which are acceptable for Geiger counter tubes, generally not in excess of from 65.degree. F. to 150.degree. F. and preferably at the lower end of such range.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing an apparatus for shielding components within a well logging instrument while also maintaining a suitable, generally stable, thermal environment for a period of time for such components.